1. Field of the Invention
The present invention relates to a safety element for a battery, which is provided with material having a Metal-Insulator Transition (MIT) characteristic where resistance abruptly drops at or above a certain temperature, and to a battery with such a safety element.
2. Description of the Related Art
In recent, active interest has been taken in energy storage technologies. Of them, an effort to research and develop a battery becomes more and more materialized as its application field is expanded into a mobile phone, a camcorder and a notebook as well as even into an electric vehicle energy source. In terms of this, an electrochemical device is the most noteworthy field, and among others, development of a secondary battery capable of being charged and discharged is now in the spotlight. From among secondary batteries in current use, a lithium ion secondary battery, which has been developed in the early 1990's, has an advantage over conventional batteries using aqueous electrolyte such as Ni-MH, Ni—Cd and H2SO4—Pb batteries in that its operating voltage is high and its energy density is extraordinarily large.
However, when a battery temperature rises in a charged state due to environmental changes such as external impact by pressure, nails or nippers, the rise of an ambient temperature, overcharging or the like, the lithium ion secondary battery is subjected to swelling caused by a reaction of electrode active material and an electrolyte and is even in danger of firing or explosion.
In particular, since positive electrode active material is sensitive to a voltage, reactivity between the positive electrode and the electrolyte is enhanced as the battery is charged and so the voltage becomes higher, which not only causes dissolution of a positive electrode surface and oxidation of the electrolyte, but also increases the risk of firing or explosion.
The more energy density is increased owing to high-capacity of a battery, in particular, a non-aqueous electrolytic secondary battery such as a lithium secondary battery, the more and more such a safety problem becomes important.